On vehicles of the kind used as tractors, in agriculture for example, or as construction vehicles, there are either no wheel brakes at all at the front axle, or these brakes are of such small dimensions that the maximum braking torque supplied thereby is not sufficient. Accordingly, the front wheels are braked entirely or primarily through the coupling to the rear wheels, which are braked by the wheel brakes. Here, the coupling between the front wheels and the rear wheels is generally accomplished by a clutch, which is closed optionally by hand or is closed automatically only during braking, coupling the rear axle driven by the power unit to a propeller shaft, which then transmits driving power or braking power to the wheels of the front axle.
German patent document DE 602 16 661 T2 discusses agricultural utility vehicles having a relatively sophisticated brake-slip-controlled brake device, said vehicles having front wheels provided with wheel brakes in addition to rear wheels provided with wheel brakes. In this document, 4S/4M configurations and 4S/3M configurations are described, in which the wheels of the front axle and the wheels of the rear axle are each provided with a wheel speed sensor, the rear axle is provided with a 2-channel brake modulator and the front axle is provided with a 2-channel or 1-channel brake modulator. The reference speed of the vehicle required for brake slip control is determined by the signals of wheel speed sensors associated with the front wheels and the rear wheels, in particular by averaging the wheel speed signals.
However, such configurations of slip-controlled brake devices are not suitable for vehicles of the type in question, such as agricultural utility vehicles (tractors or farm tractors) or construction vehicles, which have wheel brakes only at the rear axle or rear axles but not at the front axle or have brakes of only small dimensions at the front axle. However, since even such vehicles are nowadays reaching ever higher speeds, high stability and minimum braking distances when braking are a worthwhile aim. For this reason, slip-controlled braking in the context of ABS (antilock system) is desirable on such vehicles. In the case of known ABS systems, however, it is necessary to determine or estimate the vehicle reference speed in order to calculate the brake slip at the individual wheels.
However, if the front wheels are coupled to the rear wheels during braking on the above types of vehicle, there is the problem that all the vehicle wheels rotate at substantially the same wheel speed and may lock up, but the vehicle is still in motion. For this reason, the signals of the wheel speed sensors cannot supply any information on the vehicle reference speed.